Thesis (Ph.D)

Looking through the crowded mask: investigating the effect of distractor number and position in object substitution masking


Object substitution masking (OSM) is a phenomenon wherein a surrounding mask (typically four dots) that onsets with a target but lingers after its offset significantly reduces target perceptibility. OSM was originally postulated to occur only when spatial attention was spread (Di Lollo et al., 2000). Specifically, it was claimed that OSM only occurred when the target was presented in the context of large set-size displays (Di Lollo et al., 2000). However, more recent research has raised questions over the relevance of set size in OSM. Two separate investigations (Argyropoulos et al., 2013; Filmer et al., 2014) found that strong masking by OSM could be produced even with a set size of one. It was argued that the “set size” effects in OSM were actually an artifact of constrained performance. That is, once performance was brought within a measurable range, OSM was reported to be independent of set size. Further research however has suggested that perhaps this rejection of the role of set size in OSM was premature. Pilling (2013) found that increased set size did in fact lead to greater OSM magnitude. Therefore it seems that an explanation of constrained performance cannot fully account for the experimental findings. This thesis begins by investigating the disparity between these results by further exploring the role of set size in OSM. The first chapter provides an overview of some of the constraints for perceptual awareness by examining experimental phenomena that prevent visual awareness. The experimental phenomena of visual masking and specifically OSM are focused on with particular focus given to the role of attention in OSM. Chapter 2 is the first experimental chapter. This chapter investigates the role of set size in OSM using five experiments. Chapter 3 explores if visual crowding can be used as an alternative explanation for the set size effects in OSM with five experiments. Chapter 4 attempts to investigate the neural underpinnings of OSM, and the interaction between OSM and crowding using an EEG method. This thesis proposes, based on its findings, that the nominal set size effect in OSM is actually an effect of crowding, a factor which tends to co-vary with set size in most studies. Further experiments in this thesis showed that the interaction between crowding and OSM was one in which OSM affected crowding rather than the converse process. That is, with the use of OSM, the window at which flankers crowd the target becomes extended. These findings show parallels with the previously reported phenomenon of “supercrowding” which has been reported with classical masking. Given this, these results challenge claims regarding the position of OSM and crowding in the object processing hierarchy (e.g. Breitmeyer, 2014). This thesis contributes to the ongoing investigation of OSM, provides implications for its existing theories and for accounts of object processing more generally as well as highlighting future directions for research in this field.

Attached files


Camp, Sarah-Jayne


Supervisors: Pilling, Michael (0000-0002-1984-1497); Wiggs, Luci (0000-0002-5697-6550); Kumar, Sanjay (0000-0001-5018-4184)

Oxford Brookes departments

Faculty of Health and Life Sciences
Department of Psychology, Social Work and Public Health


Year: 2015

© Camp, Sarah-Jayne
Published by Oxford Brookes University
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